Supersonic pulse, plasma sampling mass spectrometry is described, with an emphasis on the physical mechanism by which species originally within the plasma are incorporated into the supersonically expanding noble gas pulse. This new method is based on the release of a short burst of noble gas into the high vacuum environment of an ECR-microwave plasma. Upon expansion through the plasma region, species originally present in the plasma become incorporated in the noble gas pulse and are detected by quadrupole mass spectrometry. The mechanism of the incorporation process is investigated through measurement of the time-of-flight velocity distributions of both the noble gas and species incorporated into the pulse. Incorporation is shown to be the result of supercooled noble gas clustering around the incorporated species, which act as nucleation sites for the condensation. It is this unique sampling method which makes this technique capable of providing a chemical snapshot of the plasma composition. Practical applications of this technique include the investigation of the composition of diamond deposition plasmas and the etching of silicon with chlorine. The investigations of diamond plasmas include the observation of a plasma that contains at least 40% of the radical species C2H3.